Technical Abstract:
Leafminer (Liriomyza spp.) is a major insect pest of many important agricultural crops including spinach (Spinacia oleracea). Chemical control is not long lasting, and leafminers can develop a high degree of resistance to insecticides. The purposes of the present experiments were to evaluate differences in spinach genotypes to leafminer damage, to compare results obtained from insect cages and from the field, and to study the association among different resistant traits. We screened 345 spinach genotypes from the USDA germplasm collection for leafminer resistance in an outdoor insect cage and in the field. Significant genotypic differences were found for leafminer stings per unit leaf area, mines per plant, and mines per 100 g plant weight. The sting result from the field was highly correlated (r = 0.770) with the result from the insect cage, demonstrating that a cage test could be used to screen for leafminer resistance in the field. Mines per plant were not correlated with plant weight, suggesting that leafminer flies did not lay their eggs randomly and oviposition-nonpreference occurred in these plants. Stings per unit leaf area was not correlated with mines per plant or per 100 g plant weight, which suggests that feeding non-preference does not necessarily mean oviposition-nonpreference for a spinach genotype and these two traits can be improved independently. A field screening of 441 genotypes from CGN (Holland) and IPK (Germany) spinach collections also identified lines with partial resistance. These findings suggest that genetic improvement of spinach for leafminer resistance is feasible. A phenotypic recurrent selection method is used to increase the level of leafminer resistance in spinach.